Kyriakos D. Papadopoulos

A slotted plate device for measuring static yield stress

A slotted-plate device was constructed with a balance and a linear-motion platform to directly measure static yield stresses of suspensions by moving the plate in the suspension in a similar mode as is done in the well-known Wilhelmy-plate technique for measuring surface tension. Wall effects associated with the original plate yield-stress instrument [De Kee et al. (1980)] were minimized by opening a series of slots on the plates. Yield-stress experiments were conducted on both high-concentration (40, 50, 60, and 70 wt % TiO2) and low-concentration (2, 3, and 5 wt % bentonite) aqueous suspensions. The new setup avoids the disadvantages of the vane instrument, possible secondary flow between the blades as well as a nonuniform stress distribution along a virtual cylindrical surface. Yield stress values of TiO2 suspensions were compared with the values obtained via a variety of other methods, including indirect extrapolation from steady-shear data, vane creep testing, and vane stress-ramp measurements using ...

Visualization of water transport in W1/O/W2 emulsions

Abstract Water transport between two aqueous phases and through an intervening oil phase under osmotic pressure was observed and quantified visually using capillary video-microscopy. Under certain conditions, a new mechanism was observed directly, according to which the pure-water phase emulsifies spontaneously and the resulting emulsified droplets migrate to the saline aqueous phase. Another finding was the importance of the thickness of the oil phase, O, that separates two aqueous bodies, W1 and W2, in that it determines which transport mechanism of water between W1 and W2 will be predominant. Specifically, in a W1/O/W2 emulsion globule where W1 represents the internal pure-water droplets and W2 the suspending saline-water medium, when W1 and W2 were at visual contact, water transport occurred mainly through the hydrated surfactant mechanism. In the case of a visible minimum distance of separation between W1 and W2, measuring from a few to over 100 μm, the water transport rate was found to be significantly lower than the rate at visual contact and water migration occurred via spontaneously emulsified droplets and reverse micelles. In all cases, the transport rate was independent of the size of the water droplets and the oil globule, and in the case of no visual contact, it was also independent of the minimum separation distance between W1 and W2. This result implies that, under the experimental conditions used, the water transport rate in W1/O/W2 emulsions is controlled by interfacial processes, rather than being diffusion controlled as has been suggested by previous work.

AFM on humic acid adsorption on mica

Abstract Atomic force microscopy (AFM) was used to measure the thickness of (purified) Aldrich humic acid (HA) molecular layers adsorbed on mica. It was found that the drying methods used in preparing a sample affect the configuration of HA molecules adsorbed on micas surface and the overall shape of the adsorbed HA layers. Adsorbed HA exhibited a more flexible configuration in freeze-dried samples than in air-dried samples. For the freeze-dried samples, results showed that 90% of HA islands have a height of 4.2–5.7 nm, while the similar range of the air-dried samples is 3.1–3.7 nm. Based on results of bearing and particle-size analyses, it is proposed that HA molecules adsorb at discrete sites to form monolayer islands at the mica/water interface. Assuming a cylindrical polyelectrolyte configuration for the HA molecules, their average diameter in solution is expected to fall within the freeze-dried height range.

Stability of water-in-oil-in-water type globules

Abstract A method for producing a single water-in-oil-in-water (W/O/W) type globule is used to study its stability microscopically, by bringing the internal water droplets in contact with each other and with the external interface between water and oil. Each experiment lasted 24 h unless there was rupture within a shorter period, and the parameters that were studied are pH, ionic strength and the concentration of Span 80 surfactant. Under all experimental conditions that produced limited stability, the internal water droplets did not coalesce among themselves, and when the globule broke it was because the internal water droplets coalesced with the external phase. In the absence of surfactant the globule was found intrinsically unstable at all pHs and salinities. Experimental results are explained through a model for the interaction energy, van der Waals and Coulombic, between a single internal water droplet and the external aqueous phase as well as between two internal water droplets. A key parameter in the model is the ‘effective thickness’ of the adsorbed layer of surfactant, which determines whether the net interaction energy is attractive or repulsive.

A method for modeling the interactions of parallel flat plate systems with surface features

Abstract A method is presented for modeling the unretarded van der Waals and the electric double layer interactions of two parallel flat plates of which one is smooth and the other is roughened, based on the methods of Hamaker and Derjaguin. The electric double layer interactions will be described for the cases of the constant surface charge and constant surface potential. The method consists of replacing the original system with flat plates which have an equivalent interaction energy. The configuration of the two flat plate system is determined by using the equivalent distance ratio. The equivalent distance ratio is the distance between the equivalent flat plates, divided by the actual plate separation. It is dependent on the nondimensional geometric parameters of the asperity system, the shape of the asperity and the type of interaction. The range of the ratio is from zero to unity for positive asperities, though for negative asperities they always exceed unity. Both positive and negative asperities are studied. In addition to presenting the equivalent distance ratio, the original model of Herman and Papadopoulos (J. Colloid Interface Sci. 136, 385, 1990 ) is used for constant base area analysis as well as for a distribution of asperities on the surface.

Effects of asperities on the van der Waals and electric double-layer interactions of two parallel flat plates

Abstract The role of geometric surface features on the van der Waals and electric double-layer interactions of two parallel flat plates is studied. Equations are developed using the Hamaker metod for van der Waals forces, and Derjaguins method for electric double-layer forces. The surface features are limited to spherical and conical asperities which include truncated asperities as well. The importance of asperities in surface interactions is assessed by using the interaction ratio, defined as the ratio of the interaction energy of a sysem with an asperity to that of the system without one.

A method for measuring bacterial chemotaxis parameters in a microcapillary.

A new method was developed which enables chemotaxis parameters to be measured at a single-cell level inside a capillary for the first time. The chemotaxis chamber consists of two reservoirs communicating through a capillary tube 50 mum in diameter. Chemotaxis parameters are measured inside the capillary using image analysis, after a nearly linear attractant concentration gradient has been generated along the capillary by diffusion. Compared to previously published techniques, this method provides a well-characterized chemoattractant concentration profile in addition to allowing single-cell parameters to be measured inside a fine capillary. This procedure was used to measure the single-cell chemotaxis parameters for Escherichia coli K12, and the results are compared to published data on single E. coli cells chemotaxing in bulk. (c) 1996 John Wiley & Sons, Inc.

Differentiation Kinetics of in Vitro 3T3-L1 Preadipocyte Cultures

Engineering autologous adipose constructs from cell culture is a promising strategy to overcome limitations of conventional soft-tissue implants. A methodology is presented to experimentally determine and mathematically model the differentiation kinetics of in vitro 3T3-L1 preadipocyte cultures that can aid in construct design. Relative rates of morphological and interfacial events during adipogenesis were compared. Model results suggest that maturation of an intermediate multilocular phenotype was the rate-limiting step in morphological differentiation and had an intrinsic rate of 0.012 day(-1). Dislodgment of multilocular fat cells was the primary mechanism of cell loss during adipogenesis. The maximum rate of lipid droplet nucleation was predicted to precede that of coalescence by 10 days and to be three times faster. Coalescence probability was estimated to decrease from 33 to 11% for 4- and 8-microm-diameter droplets, respectively. Fluid drainage and the cytoskeleton between droplets could have impeded coalescence. The kinetic analysis suggests that droplet ripening was the dominant mechanism of lipid production. Applications of this research include engineering of an adipose construct and predicting surgical outcome of patients requiring soft-tissue augmentation.

Temperature-Induced Protein Release from Water-in-Oil-in-Water Double Emulsions

A model water-in-oil-in-water (W1/O/W2) double emulsion was prepared by a two-step emulsification procedure and subsequently subjected to temperature changes that caused the oil phase to freeze and thaw while the two aqueous phases remained liquid. Our previous work on individual double-emulsion globules1 demonstrated that crystallizing the oil phase (O) preserves stability, while subsequent thawing triggers coalescence of the droplets of the internal aqueous phase (W1) with the external aqueous phase (W2), termed external coalescence. Activation of this instability mechanism led to instant release of fluorescently tagged bovine serum albumin (fluorescein isothiocyanate (FITC)-BSA) from the W 1 droplets and into W2. These results motivated us to apply the proposed temperature-induced globule-breakage mechanism to bulk double emulsions. As expected, no phase separation of the emulsion occurred if stored at temperatures below 18 degrees C (freezing point of the model oil n-hexadecane), whereas oil thawing readily caused instability. Crucial variables were identified during experimentation, and found to greatly influence the behavior of bulk double emulsions following freeze-thaw cycling. Adjustment of these variables accounted for a more efficient release of the encapsulated protein.

Electroosmotic flow through porous media: cylindrical and annular models

Abstract Electroosmotic flow rates through porous media are predicted through the cylindrical and annular geometrical models. Differences in the results obtained from the two models are compared and discussed by making reference to the electrokinetic remediation of contaminated soil. In the annular-model case, when the electrical potential on the outer wall is less than on the inner wall and/or the double-layer thickness is much shorter than the outer radius, a maximum flux is predicted at some outer-to-inner-radii ratio. At a given soil porosity, the cylindrical model predicts a higher flow rate than the annular model, and the difference becomes increasingly more pronounced as the radii ratio approaches unity, which corresponds to a low-porosity soil.